| Osteonectin is an acidic, noncollagenous glycoprotein (Mr=29,000) originally isolated from fetal and adult bovine bone matrix (1-5). In vitro bovine bone osteonectin binds type I collagen, calcium (Kd=3x10(-7)M) and hydroxyapatite (Kd=8x10(-8)M) and has been shown to be a potent inhibitor of hydroxyapatite seeded crystal growth (6). In this context it has been suggested that osteonectin may play an important role in the regulation of bone metabolism by binding hydroxyapatite to collagen. Recently, proteins homologous to osteonectin nave been identified in a number of cell types, most of which are associated with extracellular matrix production (7). The amino acid sequence (from cDNA sequences) of one of these proteins, human placental SPARC, is identical to human bone osteonectin. Osteonectin has also been identified as an alpha granule component of human platelets and is secreted during platelet activation. A small portion of the secreted osteonectin is expressed on the platelet cell surface in an activation-dependent manner (8). Purified platelet osteonectin is a single chain molecule which exhibits a slightly larger apparent molcular weight than that of osteonectin derived from bone (8). The NH2-terminal sequences of platelet and bone-derived osteonectin are identical, but the two proteins differ with regard to the extent of glycosylation (8-10). Human platelet osteonectin is isolated from thrombin-activated platelets as described by Kelm, et al. (8). Bovine bone osteonection is isolated from demineralized bone by the method of Romberg, et al. (2). Extinction Coefficient (1%, 1cm, A280): ~8.0 Localization: Bone, platelets, plasma (0.9 ug/ml)* (5), serum (2.6 ug/ml)** (5). Mode of action: Unknown Molecular Weight: 32,700 (human bone, from cDNA) (1) 29,000 (bovine bone, by sedimentation equilibrium) (3) Isoelectric Point: 5.5*** (3) Structure: Single chain, NH2 terminal acidic domain, cysteine rich serpin homology domain, 2 EF hand domains. Post-translational Modifications: Phosphoserine (11) Notes: * Determined for Human Plasma ** Determined for Human Serum *** Determined for Bovine Bone Storage and Stability: Aliquot to avoid repeated freezing and thawing. Store at -70C. Aliquots are stable for 6 months after receipt at -70C. For maximum recovery of product, centrifuge the original vial after thawing and prior to removing the cap. Further dilutions can be made in assay buffer. Caution: Although the source material tested negative for HBsAg, anti-HBcore, Syphilis, anti-HIV1/2, anti-HCV 2.0 and anti-HTLV1/2, this product should be handled as though infectious, using appropriate FDA/NIH guidelines and procedures. |
